Laser Induced Damage Threshold Tutorial

The following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When choosing optics, it is important to understand the Laser Induced Damage Threshold (LIDT) of the optics being used. The LIDT for an optic greatly depends on the type of laser you are using. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption either in the coating or in the substrate). Pulsed lasers, on the other hand, often strip electrons from the lattice structure of an optic before causing thermal damage. Note that the guideline presented here assumes room temperature operation and optics in new condition (i.e., within scratch-dig spec, surface free of contamination, etc.). Because dust or other particles on the surface of an optic can cause damage at lower thresholds, we recommend keeping surfaces clean and free of debris. For more information on cleaning optics, please see our Optics Cleaning tutorial.

Testing Method

Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.

First, a low-power/energy beam is directed to the optic under test. The optic is exposed in 10 locations to this laser beam for 30 seconds (CW) or for a number of pulses (pulse repetition frequency specified). After exposure, the optic is examined by a microscope (~100X magnification) for any visible damage. The number of locations that are damaged at a particular power/energy level is recorded. Next, the power/energy is either increased or decreased and the optic is exposed at 10 new locations. This process is repeated until damage is observed. The damage threshold is then assigned to be the highest power/energy that the optic can withstand without causing damage. A histogram such as that below represents the testing of one BB1-E02 mirror.

The photograph above is a protected aluminum-coated mirror after LIDT testing. In this particular test, it handled 0.43 J/cm² (1064 nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.

According to the test, the damage threshold of the mirror was 2.00 J/cm² (532 nm, 10 ns pulse, 10 Hz, Ø0.803 mm). Please keep in mind that these tests are performed on clean optics, as dirt and contamination can significantly lower the damage threshold of a component. While the test results are only representative of one coating run, Thorlabs specifies damage threshold values that account for coating variances.

Continuous Wave and Long-Pulse Lasers

When an optic is damaged by a continuous wave (CW) laser, it is usually due to the melting of the surface as a result of absorbing the laser's energy or damage to the optical coating (antireflection) [1]. Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions.

When pulse lengths are between 1 ns and 1 µs, laser-induced damage can occur either because of absorption or a dielectric breakdown (therefore, a user must check both CW and pulsed LIDT). Absorption is either due to an intrinsic property of the optic or due to surface irregularities; thus LIDT values are only valid for optics meeting or exceeding the surface quality specifications given by a manufacturer. While many optics can handle high power CW lasers, cemented (e.g., achromatic doublets) or highly absorptive (e.g., ND filters) optics tend to have lower CW damage thresholds. These lower thresholds are due to absorption or scattering in the cement or metal coating.

Pulsed lasers with high pulse repetition frequencies (PRF) may behave similarly to CW beams. Unfortunately, this is highly dependent on factors such as absorption and thermal diffusivity, so there is no reliable method for determining when a high PRF laser will damage an optic due to thermal effects. For beams with a high PRF both the average and peak powers must be compared to the equivalent CW power. Additionally, for highly transparent materials, there is little to no drop in the LIDT with increasing PRF.

In order to use the specified CW damage threshold of an optic, it is necessary to know the following:

1. Wavelength of your laser
2. Beam diameter of your beam (1/e²)
3. Approximate intensity profile of your beam (e.g., Gaussian)
4. Linear power density of your beam (total power divided by 1/e² beam diameter)

Thorlabs expresses LIDT for CW lasers as a linear power density measured in W/cm. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size, as demonstrated by the graph to the right. Average linear power density can be calculated using the equation below. 

The calculation above assumes a uniform beam intensity profile. You must now consider hotspots in the beam or other non-uniform intensity profiles and roughly calculate a maximum power density. For reference, a Gaussian beam typically has a maximum power density that is twice that of the uniform beam (see lower right).

Now compare the maximum power density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately. A good rule of thumb is that the damage threshold has a linear relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 10 W/cm at 1310 nm scales to 5 W/cm at 655 nm):

While this rule of thumb provides a general trend, it is not a quantitative analysis of LIDT vs wavelength. In CW applications, for instance, damage scales more strongly with absorption in the coating and substrate, which does not necessarily scale well with wavelength. While the above procedure provides a good rule of thumb for LIDT values, please contact Tech Support if your wavelength is different from the specified LIDT wavelength. If your power density is less than the adjusted LIDT of the optic, then the optic should work for your application. 

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. The damage analysis will be carried out on a similar optic (customer's optic will not be damaged). Testing may result in additional costs or lead times. Contact Tech Support for more information.

Pulsed Lasers

As previously stated, pulsed lasers typically induce a different type of damage to the optic than CW lasers. Pulsed lasers often do not heat the optic enough to damage it; instead, pulsed lasers produce strong electric fields capable of inducing dielectric breakdown in the material. Unfortunately, it can be very difficult to compare the LIDT specification of an optic to your laser. There are multiple regimes in which a pulsed laser can damage an optic and this is based on the laser's pulse length. The highlighted columns in the table below outline the relevant pulse lengths for our specified LIDT values.

Pulses shorter than 10^-9 s cannot be compared to our specified LIDT values with much reliability. In this ultra-short-pulse regime various mechanics, such as multiphoton-avalanche ionization, take over as the predominate damage mechanism [2]. In contrast, pulses between 10^-7 s and 10^-4 s may cause damage to an optic either because of dielectric breakdown or thermal effects. This means that both CW and pulsed damage thresholds must be compared to the laser beam to determine whether the optic is suitable for your application.

When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:

1. Wavelength of your laser
2. Energy density of your beam (total energy divided by 1/e² area)
3. Pulse length of your laser
4. Pulse repetition frequency (prf) of your laser
5. Beam diameter of your laser (1/e² )
6. Approximate intensity profile of your beam (e.g., Gaussian)

The energy density of your beam should be calculated in terms of J/cm². The graph to the right shows why expressing the LIDT as an energy density provides the best metric for short pulse sources. In this regime, the LIDT given as an energy density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now adjust this energy density to account for hotspots or other nonuniform intensity profiles and roughly calculate a maximum energy density. For reference a Gaussian beam typically has a maximum energy density that is twice that of the 1/e² beam.

Now compare the maximum energy density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately [3]. A good rule of thumb is that the damage threshold has an inverse square root relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm² at 1064 nm scales to 0.7 J/cm² at 532 nm):

You now have a wavelength-adjusted energy density, which you will use in the following step.

Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT [4]. For data presented here, a <1 mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the larger size beam exposing more defects.

The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1 - 100 ns, an approximation is as follows:

Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10^-9 s and 10^-7 s. For pulses between 10^-7 s and 10^-4 s, the CW LIDT must also be checked before deeming the optic appropriate for your application.

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information.

[1] R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
[2] Roger M. Wood, Laser-Induced Damage of Optical Materials (Institute of Physics Publishing, Philadelphia, PA, 2003).
[3] C. W. Carr et al., Phys. Rev. Lett. 91, 127402 (2003).
[4] N. Bloembergen, Appl. Opt. 12, 661 (1973).

Polarizer Selection Guide

Thorlabs offers a diverse range of polarizers, including wire grid, film, calcite, alpha-BBO, rutile, and beamsplitting polarizers. Collectively, our line of wire grid polarizers offers coverage from the visible range to the beginning of the Far-IR range. Our nanoparticle linear film polarizers provide extinction ratios as high as 100 000:1. Alternatively, our other film polarizers offer an affordable solution for polarizing light from the visible to the Near-IR. Next, our beamsplitting polarizers allow for use of the reflected beam, as well as the more completely polarized transmitted beam. Finally, our alpha-BBO (UV), calcite (visible to Near-IR), rutile (Near-IR to Mid-IR), and yttrium orthovanadate (YVO₄) (Near-IR to Mid-IR) polarizers each offer an exceptional extinction ratio of 100 000:1 within their respective wavelength ranges. 

To explore the available types, wavelength ranges, extinction ratios, transmission, and available sizes for each polarizer category, click More [+] in the appropriate row below.

Polarizer Type	Wavelength Range	Extinction Ratio	Transmissiona	Available Sizes
Wire Grid Polarizers on Glass Substrates	420 nm - 700 nm	>800:1 (Avg. Over Wavelength Range)		12.5 mm x 12.5 mm, Ø25.0 mmb, 25.0 mm x 25.0 mm, and 50.0 mm x 50.0 mm
	250 nm - 4 µm	>10:1 from 250 nm - 4 µm >100:1 from 300 nm - 4 µm >1000:1 from 600 nm - 4 µm >10 000:1 from 2.25 µm - 4 µm		12.5 mm x 12.5 mm, Ø25.0 mmb, 25.0 mm x 25.0 mm, and 50.0 mm x 50.0 mm
Wire Grid Polarizing Beamsplitter Cubes (Unmounted, 16 mm Cage Cube, or 30 mm Cage Cube)	400 nm - 700 nm	>1 000:1 (AOI: 0° - 5°) >100:1 (AOI: 0° - 25°)	P-Pol. S-Pol.	20 mm and 1"f
Holographic Wire Grid Polarizers	2 µm - 12 µm	150:1 at 3 µm 300:1 at 10 µm		Ø25.0 mmb and Ø50.0 mmb
	2 µm - 9 µm	150:1 at 3 µm 300:1 at 8 µm
	2 µm - 30 µm	150:1 at 3 µm 300:1 at 15 µm
	2 µm - 18 µm	150:1 at 3 µm 300:1 at 10 µm
MIR Wire Grid Polarizers on Silicon Substrates	3 µm - 5 µm	>1000:1		12.5 mm x 12.5 mmb, Ø25.0 mmb, 25.0 mm x 25.0 mmb, and 50.0 mm x 50.0 mmb
	7 µm - 15 µm	>10,000:1		12.5 mm x 12.5 mmb, Ø25.0 mmb, 25.0 mm x 25.0 mmb, and 50.0 mm x 50.0 mmb

Polarizer Type	Wavelength Range	Extinction Ratio	Transmissiona	Available Sizes
Nanoparticle Linear Film Polarizers	365 nm - 395 nm	>1000:1 from 365 nm - 395 nm >10 000:1 from 369 nm - 390 nm >100 000:1 from 372 nm - 388 nm		Ø12.5 mmc and Ø25.0 mmd
	480 nm - 550 nm	>10 000:1 from 480 nm - 550 nm		Ø12.5 mmc and Ø25.0 mmd
	510 nm - 800 nm	>1000:1 from 510 nm - 800 nm >10 000:1 from 520 - 740 nm >100 000:1 from 530 - 640 nm		Ø12.5 mmc and Ø25.0 mmd
	550 nm - 1500 nm	>10 000:1 from 550 nm - 1500 nm >100 000:1 from 600 nm - 1200 nm		Ø12.5 mmc and Ø25.0 mmd
	650 nm - 1100 nm	>200:1 from 650 nm - 695 nm >1000:1 from 695 nm - 1100 nm		Ø1/2"c and Ø1"d
	650 nm - 2000 nm	>1000:1 from 650 nm - 2000 nm >10 000:1 from 750 nm - 1800 nm >100 000:1 from 850 nm - 1600 nm		Ø12.5 mmc and Ø25.0 mmd
	1 µm - 3 µm	>1000:1 from 1 µm - 3 µm >10,000:1 from 1.2 µm - 3 µm		Ø12.5 mmc and Ø25.0 mmd
	1.1 µm - 1.8 µm	>1000:1 from 1.1 µm - 1.8 µm		Ø1/2"c and Ø1"d
	1.5 µm - 5 µm	>1000:1 from 1.5 µm - 5 µm >10 000:1 from 2 µm - 4.5 µm		Ø12.5 mmc and Ø25.0 mmd
Economy Film Polarizers	400 nm - 700 nm	>100:1 from 400 nm - 500 nm >1000:1 from 500 nm - 700 nm >5000:1 from 530 nm - 690 nm		2" x 2"
	600 nm - 1100 nm	>400:1 from 600 nm - 1100 nm >1000:1 from 600 nm - 940 nm		2" x 2"
Economy Laminated Film Polarizers	400 nm - 700 nm	>100:1 from 400 nm - 500 nm >1000:1 from 500 nm - 700 nm >5000:1 from 530 nm - 690 nm		Ø1/2", Ø1", and Ø2"
	600 nm - 1100 nm	>1000:1 from 600 nm - 950 nm >400:1 from 600 nm - 1100 nm		Ø1/2", Ø1", and Ø2"
	1050 nm - 1700 nm	>1000:1 from 1050 - 1400 nm >2000:1 from 1400 - 1700 nm		Ø1/2", Ø1", and Ø2"

Polarizer Type	Wavelength Range	Extinction Ratio	Transmissiona	Available Sizes
Polarizing Plate Beamsplitters	405 nm	>10 000:1		Ø1" and 25 mm x 36 mm
	532 nm
	633 nm
	780 nm
	808 nm
	1030 nm
	1064 nm
	1310 nm
	1550 nm
Polarizing Bandpass Filters	355 nm +6 nm / -9 nm	1 000 000:1		25.2 mm x 35.6 mm
Broadband Polarizing Beamsplitter Cubes (Unmounted, 16 mm Cage Cube, or 30 mm Cage Cube)	420 nm - 680 nm	1000:1e		5 mm, 10 mm, 1/2", 20 mmf, 1"f, and 2"
	620 nm - 1000 nm
	900 nm - 1300 nm
	1200 nm - 1600 nm
Wire Grid Polarizing Beamsplitter Cubes (Unmounted, 16 mm Cage Cube, or 30 mm Cage Cube)	400 nm - 700 nm	>1 000:1 (AOI: 0° - 5°) >100:1 (AOI: 0° - 25°)	P-Pol. S-Pol.	10 mm, 20 mmf, and 1"f
Laser-Line Polarizing Beamsplitter Cubes (Unmounted or 30 mm Cage Cube)	532 nm	3000:1		10 mm, 1/2", 1"f
	633 nm
	780 nm
	980 nm			1"f
	1064 nm			10 mm, 1/2", 1"f
	1550 nm
High-Power Laser-Line Polarizing Beamsplitter Cubes (Unmounted or 30 mm Cage Cube)	355 nm	2000:1		1/2" and 1"f
	405 nm
	532 nm
	633 nm
	780 - 808 nm
	1064 nm
Calcite Beam Displacers	350 nmg - 2.3 µm (Uncoated)	-		10 mmb (Clear Aperture, Square)
Yttrium Orthovanadate (YVO4) Beam Displacers	488 nm - 3.4 µm (Uncoated)	-		>3 mm x 5 mm Ellipseh (Clear Aperture)
	2000 nm (V Coated)

Polarizer Type	Wavelength Range	Extinction Ratio	Transmissiona	Available Sizes
alpha-BBO Glan-Laser Polarizers	210 nm - 450 nm	100 000:1		5 mmb and 10 mmb (Clear Aperture, Square)
	220 nm - 370 nm
	405 nm
Wollaston Prism Polarizers	190 nm - 3500 nm			Ø10 mmb (Clear Aperture)

Polarizer Type	Wavelength Range	Extinction Ratio	Transmissiona,h	Available Sizes
Glan-Laser Calcite Polarizers	350 nmg - 2.3 µm (Uncoated)	100 000:1		5 mmb, 10 mmj, and 15 mmb (Clear Aperture, Square)
	350 nmg - 700 nm (A Coated)
	650 nm - 1050 nm (B Coated)
	1050 nm - 1700 nm (C Coated)
	1064 nm (V Coated)j
Glan-Taylor Calcite Polarizers	350 nmg - 2.3 µm (Uncoated)			5 mmb, 10 mmb, and 15 mmb (Clear Aperture, Square)
	350 nmg - 700 nm (A Coated)
	650 nm - 1050 nm (B Coated)
	1050 nm - 1700 nm (C Coated)
Glan-Thompson Calcite Polarizers (Unmounted or Mounted)	350 nmg - 2.3 µm (Uncoated)			5 mmk and 10 mmk (Clear Aperture, Square)
	350 nmg - 700 nm (A Coated)
	650 nm - 1050 nm (B Coated)
Double Glan-Taylor Calcite Polarizers	350 nmg - 2.3 µm (Uncoated)			9 mmb (Clear Aperture, Square)
Beam Displacers	350 nmg - 2.3 µm (Uncoated)			10 mmb (Clear Aperture, Square)
Wollaston Prism Polarizers	350 nmg - 2.3 µm (Uncoated)			Ø10 mmk (Clear Aperture)
	350 nmg - 700 nm (A Coated)
	650 nm - 1050 nm (B Coated)

Polarizer Type	Wavelength Range	Extinction Ratio	Transmissiona	Available Sizes
Wollaston Prism Polarizers	400 nm - 2000 nm	10 000:1		Ø10 mmb (Clear Aperture)

Polarizer Type	Wavelength Range	Extinction Ratio	Transmissiona	Available Sizes
Wollaston Prism Polarizers	200 nm - 6.0 µm	10 000:1		Ø10 mmb (Clear Aperture)
Rochon Prism Polarizers				10 mm x 10 mm (Clear Aperture)

Polarizer Type	Wavelength Range	Extinction Ratio	Transmissiona	Available Sizes
Beam Displacers	488 nm - 3.4 µm (Uncoated) 2000 nm (V Coated)	100 000:1		>3 mm x 5 mm (Clear Aperture, Ellipse)
Wollaston Prism Polarizers	900 nm - 3.24 µm			Ø10 mmb (Clear Aperture)
Rochon Prism Polarizers	488 nm - 3.4 µm			10 mm x 10 mmb (Clear Aperture)

Polarizer Type	Wavelength Range	Extinction Ratio	Transmissiona	Available Sizes
Rutile TiO2 Polarizers	2.2 µm - 4 µm	100 000:1		9.1 mm x 9.5 mm x 9.5 mmb and 10.7 mm x 15.9 mm x 15.9 mmb

• Click on the graph icons in this column to view a transmission curve for the corresponding polarizer. Each curve represents one substrate sample or coating run and is not guaranteed.
• Mounted in a protective box, unthreaded ring, or cylinder.
• Available unmounted or in an SM05-threaded (0.535"-40) mount that indicates the polarization axis.
• Available unmounted or in an SM1-threaded (1.035"-40) mount that indicates the polarization axis.
• PBS519: Average T_P:T_S > 1000:1
• Available unmounted or mounted in cubes for cage system compatibility.
• Calcite's transmittance of light near 350 nm is typically around 75% (see Transmission column).
• Available unmounted or in an unthreaded Ø1/2" housing.
• The transmission curves for calcite are valid for linearly polarized light with a polarization axis aligned with the mark on the polarizer's housing.
• The 1064 nm V coating corresponds to a -C26 suffix in the item number.
• Available unmounted or mounted in a protective box or unthreaded cylinder that indicates the polarization axis.